In a photovoltaic generator of a photovoltaic system, it is useful to perform a string current measurement of the individual strings of photovoltaic generators. The knowledge of the individual string currents permits inferences to be drawn about the output of solar modules of the individual strings and is an indicator for malfunctions in the system such as the failure of individual solar modules.
For this purpose, the generator control box for each string usually has a shunt resistor built into it, at which it is possible to measure the amount of current contributed by the string line. This is in fact a widely used method, but it has a few disadvantages for which satisfactory solutions have not yet been found.
In this method, it is disadvantageous that for each individual string, the string line must be separately routed to the generator control box, which results in high costs due to the cable lengths required.
Such a centralized current measurement in a generator control box is also disadvantageous in that the thermal output released by the multiple shunt resistors as the current is being measured must be dissipated from the generator control boxes. This may possibly require the use of cooling fans, whose power consumption further reduces the overall output of the photovoltaic generator.
One reason for the string current measurement in the generator control box is that the components required for the string current measurement are accommodated in a way that protects them from moisture.
For example, with photovoltaic systems that have a large and therefore powerful photovoltaic generator or with large cable lengths between the photovoltaic generator and the generator control box, significantly higher costs must be taken into account as a result of the cable lengths that such a design requires. Furthermore, long cable lengths result in a loss for each cable.
Using current measuring modules for installation in the photovoltaic generator junction box also incurs additional costs for the user since these modules must be additionally purchased and installed in the generator junction box.